Detecting liquid dry conditions for liquefied compressed gases

ABSTRACT

A method and an apparatus are disclosed for detecting an occurrence of a liquid dry condition in a container containing a liquefied compressed gas while the gaseous phase of the liquefied compressed gas is being removed from the container over time. The apparatus includes a first sensor, a second sensor, and a computer, preferably a programmed logic controller (PLC). The first sensor senses temperature (T) inside the container and provides a signal indicative thereof. The second sensor senses pressure (P) inside the container and provides a signal indicative thereof. The computer receives signals from the first and second sensors, and determines the rates of change in the pressure (dP/dt) and the temperature (dT/dt) inside the container over time. The computer identifies an occurrence of a sudden increase in the rate of change in the temperature (dT/dt) inside the container and a substantial simultaneous occurrence of a sudden decrease in the rate of change in the pressure (dP/dt) inside the container, said substantially simultaneous occurrences indicating an occurrence of a liquid dry condition in the container. The preferred embodiment includes a third sensor for sensing ambient temperature (T a ) and for providing a signal indicative thereof. The computer receives a signal from the third sensor and accounts for a change in the ambient temperature in determining the rate of change in the temperature (dT/dt) inside the container over time.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 09/138,498filed Aug. 24, 1998, now U.S. Pat. No. 6,134,805; a further division ofdivisional application Ser. No. 09/694,199 filed Oct. 23, 2000 and acontinuation of divisional application Ser. No. 09/816,293 filed Mar.23, 2001.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

The present invention relates to a method and an apparatus fordelivering a liquefied compressed gas from a tube trailer or othersupply source to a use point, such as a semiconductor fabrication toolor facility, and in particular to a method and an apparatus fordetecting the occurrence of a liquid dry condition.

Reference is made to use of the invention for detection of an occurrenceof a liquid dry condition in high-pressure tubes of hydrogen chloride(HCl). (The tubes are elongated cylinders which are stacked one upon theother on a trailer for transportation of chemicals and gases, as is wellknown in the industrial gas industry.) However, the invention can beused in connection with other types of liquefied compressed gases andother types of containers.

High-purity HCl is used for certain semiconductor processes, such assilicon epitaxial deposition. Bulk HCl is delivered to semiconductorcustomers in tube trailers, which include multiple tubes typically ratedto 1800 psig. At 70° F., HCl exists as a compressed liquefied gas underits own vapor pressure of 629 psig. Customers draw the vapor from eachtube to feed their specific process applications, such that one tubeserves as the source of gas until it is considered to be empty, when acrossover panel then changes (or crosses over) the source to the nextavailable tube of gas.

It is desirable to determine when each HCl tube is near “empty” forseveral reasons. The customer desires to use as much HCl from each tubeas possible, since they are billed per full trailer of productdelivered, not by the amount of product that is used. It is undesirable,however, to draw product from tubes that are sufficiently empty that theproduct exists only in a gaseous phase, commonly referred to as a liquiddry condition. The liquid dry condition causes an increase in the levelsof impurities of lower volatility in the gas stream, including anincrease in moisture level, which causes corrosion. This could result inlower semiconductor yields.

A liquid dry point occurs when a pressurized liquefied compressed gas,such as HCl, in a container (such as a tube) is slowly vaporized forsaturated gaseous supply, as follows. When substantial amounts of theHCl exist in the liquid phase, the pressure of the system remainsrelatively stable during the release and delivery of the saturatedgaseous HCl because the liquid portion vaporizes with the input of heatfrom the environment. Eventually a physical state occurs where all ofthe liquid has been vaporized and the remaining HCl exists in anentirely unsaturated, gaseous phase. At precisely this moment, a liquiddry condition has occurred, wherein the pressure of the container decaysrapidly thereafter.

Moisture and volatile metallic compounds can typically increasesignificantly after the liquid dry point is reached in liquefiedcompressed gas such as HCl. When a two phase system exists in acontainer (such as tube trailer), a vapor liquid equilibrium ismaintained. Contaminants such as moisture and volatile metals have verylow partition coefficient and concentrate more in the liquid phaseleaving the vapor phase much cleaner. (Partition coefficient is theratio of the concentration of a volatile component in the gaseous phaseto its concentration in the liquid phase when the system is in vaporliquid equilibrium.) These contaminants get more concentrated as HClpreferentially vaporizes as ultra-pure product during transfer anddelivery to the use point. Upon reaching the liquid dry point, due tothe absence of the liquid phase, moisture and volatile metals are freeto pass with the delivered gas. Therefore, when liquid dry point isreached, higher than normal levels of moisture or any volatile metalsare experienced in the delivery of final gaseous product from the sourceof supply (such as tube trailers). It is therefore desirable to detectthe approach or obtaining the liquid dry condition.

Devices such as mass flow meters or mass flow totalizers have beenunreliable in HCl service and therefore cannot be used to detect liquiddry conditions based on mass balance calculations. Typically, a weighscale is used to identify when a tube is approaching a liquid drycondition. However, this usually requires leaving a nominal liquid heelin the tube, making it less than an optimal solution. Also, the purchaseand installation of a scale requires a large capital investment.

An alternate method of identifying when a tube is approaching a liquiddry condition is disclosed in U.S. Pat. No. 5,359,787 (Mostowy, et al.).Sensors, such as thermocouples and pressure transducers, are providedfor the source supply (e.g., a tube trailer) and the ambient temperatureconditions. The ambient temperature at the source supply is sensed, thetemperature of the chemical from the source supply is sensed, and therelative change in pressure over selected time intervals is sensed andtransmitted to a digital computational controller. These values arecompared against preset values for ambient temperature, source of supplytemperature and pressure indicative of the liquid dry point (gas phase),and when the sensed values exceed the prescribed preset values whichindicate the liquid dry point is reached, the controller provides anappropriate alarm signal.

U.S. Pat. No. 5,359,787 teaches that the liquid dry point also can becalculated by determining and inputting to the digital computationalcontroller the volume of the tube trailer, the weight of the trailerduring transfer of the liquefied compressed gas, and the ambienttemperature at the tube trailer, as well as the temperature of thechemical leaving the tube trailer and entering the delivery conduit.These values are compared to preset values already input into thecontroller which represent approach to substantially gas phase of thechemical (meaning that at least some small amount of chemical is stillin the liquid phase). When the sensed values meet or exceed the presetvalues so as to indicate the approach to the liquid dry point(substantially a gas phase), the system generates an alarm signal.

Neither of the methods disclosed in U.S. Pat. No. 5,359,787 foridentifying or determining the liquid dry point works as well as thepresent invention, which provides a more exact method for detecting theoccurrence of a liquid dry condition using a more quantitative approach.

It is desired to have a more cost effective, reliable method ofdetecting the occurrence of a liquid dry condition in a container ofliquefied compressed gas, such as HCl.

It is further desired to have a more cost effective, reliable method ofdelivering a high-purity industrial chemical in gaseous phase from atransport vehicle having multiple tubes which contain the chemical in aliquefied compressed gas phase. It also is desired to have an improvedtransport vehicle for delivering a high-purity industrial chemical ingas phase.

It is still further desired to direct the changing (or crossover) oftubes in a tube trailer or other bulk delivery system in an optimalmanner.

BRIEF SUMMARY OF THE INVENTION

The present invention is a method and an apparatus for detecting anoccurrence of a liquid dry condition in a container containing liquefiedcompressed gas. The present invention also includes a method and anapparatus for directing a crossover to a second supply of liquefiedcompressed gas upon the occurrence of a liquid dry condition in thecontainer. In addition, the present invention includes an improvement toa transport vehicle (e.g., a tube trailer) for delivering a high-purityindustrial liquefied compressed gas in gaseous phase and a method ofdelivering the high-purity industrial chemical in gaseous phase from thetransport vehicle.

In a first embodiment, the method of detecting an occurrence of a liquiddry condition in a container containing a liquefied compressed gas whilethe gaseous phase of the liquefied compressed gas is being removed fromthe container over time comprises multiple steps. The first step is tomeasure the pressure (P) inside the container over time. The next stepis to measure the temperature (T) inside the container over time. Thethird step is to determine a rate of change in the pressure (dP/dt)inside the container over time. The next step is to determine a rate ofchange in the temperature (dT/dt) inside the container over time. Thefinal step is to identify an occurrence of a sudden increase in the rateof change in the temperature (dT/dt) inside the container and asubstantially simultaneous occurrence of a sudden decrease in the rateof change in the pressure (dP/dt) inside the container, saidsubstantially simultaneous occurrences indicating an occurrence of aliquid dry condition in the container.

In a second embodiment, the method of detecting the occurrence of aliquid dry condition includes two additional steps. The first additionalstep is to monitor the ambient temperature (T_(a)). The secondadditional step is to account for a change in the ambient temperature indetermining the rate of change in the temperature (dT/dt) inside thecontainer over time.

A third embodiment of the invention is an apparatus for detecting anoccurrence of a liquid dry condition in a container (22 or 24)containing a liquefied compressed gas while the gaseous phase of theliquefied compressed gas is being removed from the container over time.The apparatus includes a first sensor (12), a second sensor (14) and acomputer (16). The first sensor senses temperature (T) inside thecontainer and provides a signal indicative thereof. The second sensorsenses pressure (P) inside the container and provides a signalindicative thereof. The computer receives signals from the first andsecond sensors, and determines rates of change in the pressure (dP/dt)and the temperature (dT/dt) inside the container over time. The computeralso identifies an occurrence of a sudden increase in the rate of changein the temperature (dT/dt) inside the container and a substantiallysimultaneous occurrence of a sudden decrease in the rate of change inthe pressure (dP/dt) inside the container, said substantiallysimultaneous occurrences indicating an occurrence of a liquid drycondition in the container.

In the preferred embodiment, the computer (16) is a programmed logiccontroller (PLC). The first sensor (12) preferably is a thermocouple,and the second sensor (14) preferably is a pressure transducer. Theapparatus also may include an alarm (34) to report the occurrence of aliquid dry condition.

In a fourth embodiment, the apparatus also includes a third sensor (20)for sensing ambient temperature (Ta) and for providing a signalindicative thereof. The computer receives the signal from the thirdsensor and accounts for a change in the ambient temperature indetermining the rate of change in the temperature (dT/dt) inside thecontainer overtime.

In one variation of this embodiment, the computer accounts for a changein the ambient temperature (T_(a)) by a method comprising the followingsteps: (a) receiving the signals from the first and second sensorsindicating the temperature (T) and the pressure (P) inside thecontainer; (b) calculating a change in pressure inside the containerover time ΔP_(t), a change in temperature inside the container over timeΔT_(t), and a change in ambient temperature over time ΔT_(a); (c)calculating $\frac{\Delta \quad P_{t}}{\Delta \quad t_{t}}$

and $\frac{\Delta \quad T_{t}}{\Delta \quad t_{t}},$

wherein Δt_(t) is an interval of time; (d) calculating$\frac{\Delta \quad P_{t}}{\Delta \quad T_{t}};$

(e) comparing the value of$\frac{\Delta \quad P_{t}}{\Delta \quad T_{t}}$

with a first preset range of values referring to normal runningconditions; (f) determining if the$\frac{\Delta \quad P_{t}}{\Delta \quad T_{t}}$

value is out of the first preset range of values; (g) if the$\frac{\Delta \quad P_{t}}{\Delta \quad T_{t}}$

value is out of the first preset range of values, calculating$\frac{\Delta \quad T_{t}}{\Delta \quad T_{a}}$

and $\frac{\Delta \quad P_{t}}{\Delta \quad T_{a}};$

(h) comparing the calculated values of$\frac{\Delta \quad T_{t}}{\Delta \quad T_{a}}$

and $\frac{\Delta \quad P_{t}}{\Delta \quad T_{a}}$

with a second preset range of values for normal running conditions; (i)if the calculated values of$\frac{\Delta \quad T_{t}}{\Delta \quad T_{a}}$

and $\frac{\Delta \quad P_{t}}{\Delta \quad T_{a}}$

are out of the second-preset range of values, repeating steps (a)through (i).

In a fifth embodiment, the apparatus includes a data logging device forreceiving the signals from the first and second sensor, and forconverting the signals to the measurements of pressure (P) andtemperature (T) inside the container at specific points in time. Thedata logging device also determines the rates of change in the pressure(dP/dT) and the temperature (dT/dt) inside the container over time, andrecords the measurements of pressure (P), temperature (T), and rates ofchange in the pressure (dP/dt) and the temperature (dT/dt) inside thecontainer as a function of time. The data logging device also mayreceive a signal from the third sensor, convert that signal to ameasurement of ambient temperature (T_(a)) at specific points in time,and record the ambient temperature (T_(a)) as a function of time.

A sixth embodiment of the invention is a method of directing a crossoverto a second supply (24) of liquefied compressed gas upon an occurrenceof a liquid dry condition in a container (22) containing a first supplyof liquefied compressed gas while the gaseous phase of the first supplyof the liquefied compressed gas is being received from the containerover time. The method includes multiple steps. The first step is tomeasure the pressure (P) inside the container over time. The next stepis to measure the temperature (T) inside the container over time. Thethird step is to determine a rate of change in the pressure (dP/dt)inside the container over time. The fourth step is to determine a rateof change in the temperature (dT/dt) inside the container over time. Thefifth step is to identify an occurrence of a sudden increase in the rateof change in the temperature (dT/dt) inside the container and asubstantially simultaneous occurrence of a sudden decrease in the rateof change in the pressure (dP/dt) inside the container, saidsubstantially simultaneous occurrences indicating an occurrence of aliquid dry condition in the container. The final step is to actuate acrossover to the second supply of liquefied compressed gas uponidentifying the occurrence of a sudden increase in the rate of change inthe temperature (dT/dt) inside the container and a substantiallysimultaneous occurrence of a sudden decrease in the rate of change inthe pressure (dP/dt) inside the container.

A seventh embodiment of the invention is a method of directing acrossover to the second supply of liquefied compressed gas whichincludes two additional steps. The first additional step is to monitorthe ambient temperature (T_(a)). The second additional step is toaccount for a change in the ambient temperature in determining the rateof change in the temperature (dT/dt) inside the container over time.

In one variation of this embodiment, the second additional step (i.e.,accounting for a change in the ambient temperature) comprises thefollowing sub-steps:

(a) calculating a change in pressure inside the container over timeΔP_(t), a change in temperature inside the container over time ΔT_(t),and a change in ambient temperature over time ΔT_(a); (b) calculating$\frac{\Delta \quad P_{t}}{\Delta \quad t_{t}}$

and $\frac{\Delta \quad T_{t}}{\Delta \quad t_{t}},$

wherein Δt_(t) is an interval of time; (c) calculating$\frac{\Delta \quad P_{t}}{\Delta \quad T_{t}};$

(d) comparing the value of$\frac{\Delta \quad P_{t}}{\Delta \quad T_{t}}$

with a first preset range of values referring to normal runningconditions; (e) determining if the$\frac{\Delta \quad P_{t}}{\Delta \quad T_{t}}$

value is out of the first preset range of values; (f) if the$\frac{\Delta \quad P_{t}}{\Delta \quad T_{t}}$

value is out of the first preset range of values, calculating$\frac{\Delta \quad T_{t}}{\Delta \quad T_{a}}$

and $\frac{\Delta \quad P_{t}}{\Delta \quad T_{a}};$

(g) comparing the calculated values of$\frac{\Delta \quad T_{t}}{\Delta \quad T_{a}}$

and $\frac{\Delta \quad P_{t}}{\Delta \quad T_{a}}$

with a second preset range of values for normal running conditions; (h)if the calculated values of$\frac{\Delta \quad T_{t}}{\Delta \quad T_{a}}$

and $\frac{\Delta \quad P_{t}}{\Delta \quad T_{a}}$

are out of the second preset range of values, repeating sub-steps (a)through (h).

An eighth embodiment is an apparatus for directing a crossover to asecond supply of liquefied compressed gas upon an occurrence of a liquiddry condition in the container containing a first supply of liquefiedcompressed gas while the gaseous phase of the first supply of theliquefied compressed gas is being removed from the container over time.The apparatus includes the following: (1) means (14) for measuring thepressure (P) inside the container over time; (2) means (12) formeasuring the temperature (T) inside the container over time; (3) means(16) for determining a rate of change in the pressure (dP/dt) inside thecontainer over time; (4) means (16) for determining a rate of change inthe temperature (dT/dt) inside the container over time; (5) means (16)for identifying an occurrence of a sudden increase in the rate of changein the temperature (dT/dt) inside the container and a substantiallysimultaneous occurrence of a sudden decrease in the rate of change inthe pressure (dP/dt) inside the container, said substantiallysimultaneous occurrences indicating an occurrence of a liquid drycondition in the container; and (6) means (26, 28, 30 and 32) foractuating a crossover to the second supply of liquefied compressed gasupon identifying an occurrence of a sudden increase in the rate ofchange in the temperature (dT/dt) inside the container and asubstantially simultaneous occurrence of a sudden decrease in the rateof change in the pressure (dP/dt) inside the container.

In a ninth embodiment, the apparatus for directing a crossover to asecond supply of liquefied compressed gas upon an occurrence of a liquiddry condition in the container containing a first supply of liquefiedcompressed gas also includes: (1) means for monitoring the ambienttemperature (T_(a)); and (2) means for accounting for a change in theambient temperature (T_(a)) in determining the rate of change in thetemperature (dT/dt) inside the container over time.

A tenth embodiment is a method of delivering a high-purity industrialliquefied compressed gas in gaseous phase from a transport vehiclehaving multiple tubes which contain the liquefied compressed gas ingaseous phase. The method includes multiple steps, as follows: (a)connecting a first tube (22) of the vehicle (18) to a delivery system(36); (b) discharging the liquefied compressed gas in gaseous phase overtime through the delivery system; (C) detecting the occurrence of aliquid dry condition in the first tube (22); (d) automaticallydisconnecting (30) the first tube from the delivery system upon saiddetection of the liquid dry condition; (e) connecting (32) a next tube(24) of the vehicle to the delivery system; and (f) repeating steps (b)through (e) until the liquefied compressed gas has been discharged fromall tubes of the vehicle.

In the preferred embodiment, the step of detecting the occurrence ofliquid dry condition in the tube [i.e., step (c)] comprises multiplesub-steps. The first sub-step is to measure the pressure (P) inside thetube over time. The next sub-step is to measure the temperature (T)inside the tube over time. The third sub-step is to determine a rate ofchange in the pressure (dP/dt) inside the tube over time. The fourthsub-step is to determine a rate of change in the temperature (dT/dt)inside the tube over time. The final sub-step is to identify anoccurrence of a sudden increase in the rate of change in the temperature(dT/dt) inside the tube and a substantially simultaneous occurrence of asudden decrease in the rate of change in the pressure (dP/dt) inside thetube, said substantially simultaneous occurrences indicating anoccurrence of a liquid dry condition in the tube.

An eleventh embodiment is an improvement to a transport vehicle (18) fordelivering a high-purity industrial liquefied compressed gas in gaseousphase, the vehicle being of the type having multiple tubes (22, 24)which contain the liquefied compressed gas in gaseous phase. Theimprovement includes: (1) means (30) for connecting a first tube (22) ofthe vehicle (18) to a delivery system (36); (2) means (12, 14, 16) fordischarging the liquefied compressed gas in gaseous phase over timethrough the delivery system; (3) means for detecting the occurrence of aliquid dry condition in the tube; (4) means (26) for automaticallydisconnecting the first tube (22) from the delivery system (36) uponsaid detection of the liquid dry condition; and (5) means (32) forconnecting a next tube (24) of the vehicle (18) to the delivery system(36).

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a graph showing gas temperature inside a tube of hydrogenchloride (HCl) and ambient temperature (T_(a)) over a 48-hour cycleduring which HCl gas is withdrawn from the tube;

FIG. 2 is a graph showing the pressure (P) inside a tube of hydrogenchloride (HCl) over a 48-hour cycle as HCl gas is withdrawn from thetube;

FIG. 3 is a graph showing the rate of change in temperature (dT/dt)inside a tube of hydrogen chloride (HCl) over a 48-hour cycle as HCl gasis withdrawn from the tube; and

FIG. 4 is a graph showing the rate of change in pressure (dP/dt) insidea tube containing hydrogen chloride (HCl) over a 48-hour cycle as HClgas is withdrawn from the tube.

FIG. 5 is a schematic drawing of a preferred embodiment of the apparatusof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention teaches a method and an apparatus for detectingthe liquid dry condition by monitoring the pressure and temperatureinside each tube (container) of HCl (or other liquefied compressed gas)during the delivery life of each tube (container). By understanding thethermodynamic phenomena of a tube as it departs from a vapor liquidequilibrium and approaches a liquid dry condition, it can be ascertainedwhen a tube of HCl has gone to a liquid dry condition.

Several process simulations were run with the objective of identifyingdetectable trends at the time that a container, such as a tube, runsliquid dry. The process was modeled under the assumption that the tubewall temperature was equal to the bulk temperature of the gas in thecontainer.

When simulating a tube of HCl, such that the tube is initially full,with gas withdrawn at a high flow rate, and subject to a 24-hourvariable ambient temperature cycle, the pressure (P) and temperature (T)of the system typically behave according to the graphs shown in FIGS. 1and 2.

Initially, the container contents are assumed to be in vapor liquidequilibrium at a temperature (T) very close to the ambient temperature(T_(a)). As flow begins, both the temperature (T) and pressure (P) fall.If the ambient temperature (T_(a)) is relatively hot, both of theseparameters (T, P) may tend to stabilize for a short period of time, asthe environment is able to supply enough energy in the form of heat tobalance the system. However, as nightfall arrives and the ambienttemperature (T_(a)) cools, the system will once again begin to declinein temperature (T) and pressure (P). As the ambient temperature (T_(a))begins to rise again, the system may see an increase in tube pressure(P) and temperature (T) due to the increased temperature difference(delta T). This rise will lag behind the ambient temperature rise in asomewhat predictable fashion.

The process simulation shows two common patterns that appear immediatelyafter the system has gone liquid dry. First, the temperature (T) tendsto change suddenly and dramatically in the direction of ambienttemperature (T_(a).). Since the tube temperature (T) is typically lessthan ambient, this effect shows up as a temperature increase. As thesystem continues to flow product, the bulk temperature (T) of the tubecontinues to approach ambient temperature (T_(a)), and if let run longenough, it will reach equilibrium with ambient conditions.

Immediately after running liquid dry, the system pressure (P) (thesecond measurable parameter) begins to drop faster than it had when invapor liquid equilibrium. For high flow rates, this point is immediatelyobvious. For lower flow rates, this point is less obvious, but stilldetectable.

One way to determine when an HCl tube runs liquid dry more exactly is tolook at the change in temperature (dT/dt) and pressure (dP/dt) per unittime, as shown in FIGS. 3 and 4. The value of the dT/dt term (rate ofchange in temperature) stays relatively close to zero until the systemruns liquid dry, where that term spikes substantially, as shown in FIG.3 (between 2500 and 3000 minutes). As the system continues to run andthe tube temperature (T) approaches the ambient temperature (T_(a)), thevalue of the dT/dt term gradually begins to fall back to zero. The dP/dtterm (rate of change in pressure) also changes dramatically when thesystem runs liquid dry, only in the other direction, as shown in FIG. 4(between 2500 and 3000 minutes). Since the pressure (P) falls moresharply when the system is liquid dry, the dP/dt term becomes morenegative in value. It is noted that these changes (i.e., the spikes indT/dt and dP/dt) occur at the same time.

Using a data logging device, a liquid dry condition in a tube (22 or 24)can be identified by a sudden increase in dT/dt with a simultaneoussudden decrease in dP/dt. It is important to consider common conditionsthat may either indicate a false liquid dry signal or cause a ProgrammedLogic Controller (PLC) (16) to fail to detect a true liquid dry signalaccording to the criteria discussed above. (It would be desirable to usea PLC to direct the changing of tubes in a tube trailer (18) or otherbulk delivery system in an optimal manner.)

In the case of a no flow condition, the model consistently predicts thatthe system will see a slow increase in both pressure (P) and temperature(T) due to the warming effect on the tube. This will result in anincrease of both dT/dt and dP/dt, and therefore, should not cause anyproblems. On the other hand, a sudden surge of flow will have theopposite effect, namely to show a decrease of both dT/dt and dP/dt.Again, this condition poses no problems for the PLC.

Since changing weather conditions may present a problem, ambienttemperature (T_(a)) also should be monitored. If a cold front causes adramatic and sudden drop in ambient temperature (30° F. or more) at theprecise time that a tube goes liquid dry, this would wash out the spikein dT/dt. The sudden decrease in dP/dt, however, would become morepronounced. On the other hand, a sudden increase in ambient temperatureat the precise time that a tube goes liquid dry may wash out the suddendecrease in dP/dt, but would amplify the spike in dT/dt. While theseconditions may be abnormal, they must be accounted for when programmingthe logic for tube change over (or crossover).

Any mid range PLC with basic mathematical functionality will be able toperform this task. The processor will have a floating point mathematicalcapability. The PLC will sample pressure (P) and temperature (T) in thetube and ambient temperature (T_(a)) in an interval of a fixed time(e.g., milliseconds). (As previously discussed, signals indicative of P,T, and T_(a) may be received by the PLC from the first, second, andthird sensors.) The PLC then will calculate the change in pressure overtime ΔPt, the change in temperature over time ΔT_(t), and the change inambient temperature ΔT_(a) over time. In the next step, the PLC willcalculate $\frac{\Delta \quad P_{t}}{\Delta \quad t_{t}}$

and $\frac{\Delta \quad T_{t}}{\Delta \quad t_{t}}$

(where Δt_(t) is the interval of time) and subsequently will calculate$\frac{\Delta \quad P_{t}}{\Delta \quad T_{t}}.$

It will compare the value of$\frac{\Delta \quad P_{t}}{\Delta \quad T_{t}}$

with a preset range of values referring to the normal runningconditions. If the $\frac{\Delta \quad P_{t}}{\Delta \quad T_{t}}$

value is out of range, then the PLC will calculate$\frac{\Delta \quad T_{t}}{\Delta \quad T_{a}}$

and $\frac{\Delta \quad P_{t}}{\Delta \quad T_{a}}$

and compare them with another preset range of values for normal runningconditions. If these values also are out of range, then this wholeprocedure will be repeated several times on a very close time intervalbefore reaching the conclusion as to whether T_(a) is the root cause forthese deviations or an actual liquid dry condition has occurred.

The present invention also provides an alarm feature to report by alarmthe condition of liquid dry point. The alarm may be an audible siren, avisual light, a report on a computer system, or any combination of thesetypes of alarms.

In conclusion, the process model indicates that the liquid dry point canbe detected reliably by measuring system pressures (P) and temperatures(T). The point at which a system becomes liquid dry is manifested bydetectable trends in dT/dt and dP/dt that exist in the momentsimmediately after an HCl tube has gone liquid dry, especially forsystems with very high flow rates. These trends are subject to ambienttemperature conditions, but may be predicted for a given set ofcircumstances.

Various embodiments of the present invention have been described above.However, it will be appreciated that variations and modifications may bemade to those embodiments within the scope of the appended claims.

What is claimed is:
 1. A transport vehicle for delivering a high-purityindustrial liquefied compressed gas in gaseous phase, the vehicle beingof the type having multiple tubes which contain the liquefied compressedups having an apparatus for detecting an occurrence of a liquid drycondition in said tubes containing a liquefied compressed gas while theliquefied gaseous phase of the compressed gas is being removed from saidtubes over time, comprising: a first sensor for sensing temperature (T)inside said tubes and for providing a signal indicative thereof; asecond sensor for sensing pressure (P) inside said tubes and forproviding a signal indicative thereof; and a computer for receivingsignals from the first and second sensors, determining rates of changein the pressure (dP/dt) and the temperature (dT/dt) inside over time,and identifying an occurrence of a sudden increase in the rate of changein the temperature (dT/dt) inside said tubes and a substantiallysimultaneous occurrence of a sudden decrease in the rate of change inthe pressure (dP/dt) inside said tubes, said substantially simultaneousoccurrences indicating an occurrence of a liquid dry condition in saidtubes.
 2. An apparatus for detecting an occurrence of a liquid drycondition in said tubes as in claim 1, wherein the computer is aprogrammed logic controller.
 3. An apparatus for detecting an occurrenceof a liquid dry condition in said tubes as in claim 1, furthercomprising: a third sensor for sensing ambient temperature (T_(a)) andfor providing a signal indicative thereof; and wherein the computerreceives the signal from the third sensor and accounts for a change inthe ambient temperature in determining the rate of change in thetemperature (dT/dt) inside said tubes over time.
 4. An apparatus fordetecting an occurrence of a liquid dry condition in said tubes as inclaim 1, further comprising: a data logging device for receiving thesignals from the first and second sensors, converting said signals tomeasurements of pressure (P) and temperature (T) inside said tubes atspecific points in time, determining rates of change in the pressure(dP/dT) and the temperature (dT/dt) inside said tubes over time, andrecording the measurements of pressure (P), temperature (T), and ratesof change in the pressure (dP/dt) and the temperature (dT/dt) insidesaid tubes as a function of time.
 5. An apparatus for detecting anoccurrence of a liquid dry condition in said tubes as in claim 3,further comprising: a data logging device for receiving the signals fromthe first, second and third sensors, converting said signals tomeasurements of pressure (P) and temperature (T) inside and ambienttemperature (T_(a)) at specific points in time, determining rates ofchange in the pressure (dP/dT) and the temperature (dT/dt) inside overtime, and recording the measurements of pressure (P), temperature (T),ambient temperature (T_(a)), and rates of change in the pressure (dP/dt)and the temperature (dT/dt) inside said tubes as a function of time. 6.An apparatus for detecting an occurrence of a liquid dry condition in atleast one of the tubes as in claim 1, further comprising an alarm toreport the occurrence of a liquid dry condition.
 7. An apparatus fordetecting an occurrence of a liquid dry condition in at least one of thetubes as in claim 1, wherein the first sensor is a thermocouple.
 8. Anapparatus for detecting an occurrence of a liquid dry condition in atleast one of the tubes as in claim 1, wherein the second sensor is apressure transducer.
 9. A transport vehicle for delivering a high-purityindustrial liquefied compressed gas in gaseous phase, the vehicle beingof the type having multiple tubes which contain the liquefied compressedgas having an apparatus for detecting an occurrence of a liquid drycondition in at least one of the tubes containing a liquefied compressedgas while the gaseous phase of the liquefied compressed gas is beingremoved from at least one of the tubes over time, comprising: means formeasuring pressure (P) inside at least one of the tubes over time; meansfor measuring temperature (T) inside at least one of the tubes overtime; means for determining a rate of change in the pressure (dP/dt)inside at least one of the tubes over time; means for determining a rateof change in the temperature (dT/dt) inside at least one of the tubesover time; and means for identifying an occurrence of a sudden increasein the rate of change in the temperature (dT/dt) inside at least one ofthe tubes and a substantially simultaneous occurrence of a suddendecrease in the rate of change in the pressure (dP/dt) inside at leastone of the tubes, said substantially simultaneous occurrences indicatingan occurrence of a liquid dry condition in at least one of the tubes.10. An apparatus for detecting an occurrence of a liquid dry conditionin at least one of the tubes as in claim 9, further comprising: meansfor monitoring ambient temperature (T_(a)); and means for accounting fora change in the ambient temperature in determining the rate of change inthe temperature (dT/dt) inside the at least one of the tubes over time.11. A transport vehicle for delivering a high-purity industrialliquefied compressed gas in gaseous phase, the vehicle being of the typehaving multiple tubes which contain the liquefied compressed gas havingan apparatus for directing a crossover to a second supply of liquefiedcompressed gas upon an occurrence of a liquid dry condition in at leastone of the tubes containing a first supply of a liquefied compressed gaswhile the gaseous phase of the first supply of liquefied compressed gasis being removed from the at least one of the tubes over time,comprising: means for measuring pressure (P) inside the at least one ofthe tubes over time; means for measuring temperature (T) inside the atleast one of the tubes over time; means for determining a rate of changein the pressure (dP/dt) inside the at least one of the tubes over time;means for determining a rate of change in the temperature (dT/dt) insidethe at least one of the tubes over time; means for identifying anoccurrence of a sudden increase in the rate of change in the temperature(dT/dt) inside the at least one of the tubes and a substantiallysimultaneous occurrence of a sudden decrease in the rate of change inthe pressure (dP/dt) inside the at least one of the tubes, saidsubstantially simultaneous occurrences indicating an occurrence of aliquid dry condition in the at least one of the tubes; and means foractuating a crossover to the second supply of liquefied compressed gasupon identifying an occurrence of a sudden increase in the rate ofchange in the temperature (dT/dt) inside the at least one of the tubesand a substantially simultaneous occurrence of a sudden decrease in therate of change in the pressure (dP/dt) inside the at least one of thetubes.
 12. An apparatus for directing a crossover to a second supply ofliquefied compressed gas upon an occurrence of a liquid dry condition inat least one of the tubes containing a first supply of a liquefiedcompressed gas as in claim 11, further comprising: means for monitoringthe ambient temperature (T_(a)); and means for accounting for a changein the ambient temperature in determining the rate of change in thetemperature (dT/dt) inside the at least one of the tubes over time.